18F452 datasheet(187/332 Pages) MICROCHIP

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 2002 Microchip Technology Inc.

DS39564B-page 185

PIC18FXX2

calculate

the

minimum

acquisition

time,

Equation 17-1 may be used. This equation assumes

that 1/2 LSb error is used (1024 steps for the A/D). The

1/2 LSb error is the maximum error allowed for the A/D

to meet its specified resolution.

EQUATION 17-1:

ACQUISITION TIME

EQUATION 17-2:

A/D MINIMUM CHARGING TIME

Example 17-1 shows the calculation of the minimum

required acquisition time, TACQ. This calculation is

based on the following application system assump-

tions:

• CHOLD

120 pF

• Rs

2.5 k

Ω

• Conversion Error

≤

1/2 LSb

• VDD

→ Rss = 7 kΩ

• Temperature

°C (system max.)

• VHOLD

0V @ time = 0

EXAMPLE 17-1:

CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME

TACQ

Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient

=TAMP + TC + TCOFF

VHOLD =

(VREF – (VREF/2048)) • (1 – e(-Tc/C

HOLD

(RIC + RSS + RS)))

-(120 pF)(1 k

Ω + RSS + RS) ln(1/2048)

TACQ =TAMP + TC + TCOFF

Temperature coefficient is only required for temperatures > 25

°C.

TACQ =2

µs + TC + [(Temp – 25°C)(0.05 µs/°C)]

=-CHOLD (RIC + RSS + RS) ln(1/2048)

-120 pF (1 k

Ω + 7 kΩ + 2.5 kΩ) ln(0.0004883)

-120 pF (10.5 k

Ω) ln(0.0004883)

-1.26

µs (-7.6246)

9.61

µs

TACQ =2

µs + 9.61 µs + [(50°C – 25°C)(0.05 µs/°C)]

11.61

µs + 1.25 µs

12.86

µs

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